Method and apparatus for continuously determining vapor pressure



May 21, 1935. E. P.VALBY ET AL METHOQAND APPARATUS FOR CONTINUOUSLY DETERMINING VAPOR PRESSURE Filed May 25, 1932 INVENTOQS Eve/M2 F? VALBY MAP/0N L. ARNOLD ATTORNEY mama a, .21, was

NITED STAT METHOD APPARATUS FOR CONTINU- OUSLY DETERMINING VAPOR PRESSURE Edgar P. Valby and Marion L. Arnold,

Long Beach, Calif Application May 23, 1932, Serial No. 612,980-

6Claims.

This invention has todo in a general way with the measurement of vapor pressure and is more particularly directed to a method and an apparatus whereby a continuous measurement oi the vapor pressure of a flowing stream of liquid can yield a product 01' specified vapor pressure.

- are carried out with steam, there is in such proc- His important, therefore, since these processes are largely continuous in operation, to have some means for observing the vapor pressure of the product for controlling the temperatures in the refining equipment. This has been done heretofore by taking samples of the condensate from time to time and determining the vapor pressure 01' such samples by means oi a so-called vapor pressure bomb. Such a procedure is obviously insuflicient for controlling the operation oi modern continuous refinery equipment, and it accordingly becomes a primary object of this invention to produce a method and an apparatus whereby the vapor pressure of the condensate or discharge liquid can be determined at any time from a reading obtained directly from a "flowing sample. v

In this connection it is a further object of the invention to produce a method and apparatus whereby a continuous daily record of vapor pressure measurements may be obtained for checking the operation oi the refinery and de termining he vapor pressure of the product.

Inasmuch as certain distillation procewes, es-

pecially those used in the refining oi gasoline,

cases a certain amount of water vapor in the product, and it is a further object oi this invention to develop a method for meing the vapor pressure oi the pure liquids, and correcting ior any contamination with water vapor.

The details in the process contemplated by this invention and a preferred type 0! apparatus employed in practicing the same, will behest understood from the following description of the accompanying drawing which is chosen for illustrative purposes only and which shows, diagrammatically, a preferred form of apparatus used in the practice of the invention.

This

The principle which forms the basis for the method contemplated by thisinvention is that of air equilibrium, using an orifice meter or other suitable means, such as a displacement meter, to measure the volume oi air and, vapor mixture. Essentially .the method is to bring a oonstant stream of air' into equilibrium with a continuous flow oi the liquid whose vapor pres sure is to be measured (gasoline) and measure the increase in volume of air, brought about by 10 the introduction of vapors from the liquid. This measurement can be very easily obtained from the pressure diflerentialacross an orifice placed in the path of flow.

It, thereiore, follows that our invention is 15 based upon the principle that the partial pres.- sure of a vapor in a gas-vapor. mixture bears a direct relation to the relative volume of vapor in the mixture. In other words, assume a certain volume Qm oi gas-vapor mixture having Ga 20 parts of gas (air) and Q2; parts of vapor, to have an absolute pressure P at a given temperature. Then the partial pressureoi the vapor, or the vapor pressure (VP) bears the same relation to the total pressure as the volume of vapor does to 25- the total volume or It will be seen thereiore that if thevoiume of so an air and gasoline vapor mixture for example at a given pressure is known, and the volume of air alone at the same pressure is lmo, the vapor pressure can be readily calculated. ircm Equation in) above.

Referring to the the which shows, :1:- grammatically, one form 01' apparatus which can be used for obtaining values or measurements ior use in connection with Equation (a) (or more particularly with another equation which'will 4o be derived from Equation (m) we vs at E, a constant temperature bath oi w well known which is shown as comp a1: a tank I, provided with a stirringv device 2, and equipped with a! heater 8, controlled by a thermostat 4 for 45 maintaining a constant rature.

In this connection it is to be understood that the temperature must be maintained constant so that the vapor pressure measurements are all obtained tor a predetermined temperature.

The vapor pressure apparatus itself is shown as comprising a gas-vapor equilibrium chamber. or equilibrium tube 8 which is preferably inclined and provided with a plurality oi! bends so as to provide liquid seals to'assure intimate contact between the gas and the liquid for assuring complete equilibrium. The equilibrium tube 6 is provided with an. inlet connection I which in turn is provided with an orifice connection-indicated at 8 through which a suitable gas (pref' erably air) is delivered into the equilibrium tube from the air supply pipe a. The latter is can be accomplished by using a pin-hole orifice at 8 and maintaining a pressure drop of 20 lb. per sq. inch or more across this orifice; since for pressure drops of more than 50% the volume' of air passed by an orifice is constant irrespective of changes in downstream pressure.

Reference numeral 52 indicates a liquid inlet connection which is shown as being provided with a conduit l3 leading to a source of liquid whose vapor pressure is to be determined, such as -the pipe M. Another connection i5 is provided for admitting another liquid such as water,

which may be a contamination in the first liquid. The purpose of this connection will be hereinafter more fully explained. With regard to the inlet and outlet connections, it will be understood that VP the liquid whose vapor pressure is to be measured may be passed through the equilibrium chamber in a direction opposite .to the direction oi flow. of. the gas by a slight change in the arrangement of theseconnections.

- The outlet it .of the equilibrium tube is connected with a separating vessel i? which has a liquid outlet l8 provided with a valve 49 for controlling the liquid level in the tank, and a gas outlet 20. The gas outlet 20 is provided with an orifice 2i and suitable means such as the recording pressure gauge 22 for measuring the pressure difierential across the orifice.

In operating the apparatus, the valve it is first regulated so that a constant volume of air is passing through the equilibrium tube 5. The valve is being closed, this air flows through the orifice 2i and the pressure difierential of the air (Hat) is measured. If the final vapor pressure is to be corrected for a contamination with water,

the valve 15' is opened and the value of Ha is the pressure differential for air saturated with water vapor. The valve l5 then remains open and the final vapor pressure values are automatica11y corrected for water contamination, since the air, in both the first and second differential pres sure measurements, is saturated with water vapor. In other words, there can be no change in the second pressure reading over the first due to water vapor from the gasoline as the air is saturated with water vapor in both instances. After the reading (Ha) is obtained the valve I3 is adjusted so that a stream of liquid is admitted to the stream of air. The liquid as it passes through the equilibrium tube 6, is partially vaporized to saturate the ether air and water vapor and isv brought to equilibrium before it enters the separating chamber l'l. At this point the unevaporated liquid separates and is drawn off through connection l8 and the air and vapor mixture passes through outlet 20 and orifice 2|, the pressure differential of the mixture (Hm) being recorded on the recording gauge 22.

'From the foregoing description, it will be seen that we now have as known factors:the pressure difierential of the air, or air and water vapor (Ha) across the orifice ii; the pressure differential oi the mixture (Hm) and the downstream pressure (Pi) from the orifice at which in this case is the barometric pressure.

Going back to Equation ((1) we also know that:

Qa=CaVHa /F I (b) Qm ca /E341 where Ca and Cm are the orifice coefilcients for the air, or air and water vapor, and the mixture,

respectively.

Also the absolute'pressure oi the mixture and the absolute pressure of the air, or air and water vapor,

But Q11 in Equation (a) must be corrected to the actual volume it would have at the equilibrium pressure P, then Qa (corrected) P +Ha =cavHaJ fixm Then substituting Equations (12), (c) and (d) in Equation (a) we have NOW Cm Ca T E-r Where G is the specific gravity of the vapor mixture with respect to air, or the gas for which Co is the orifice coemcient. Then substituting in (f) From the foregoing description, it will be seen that so long as the volume of air passing through the apparatus is maintained constant, and the liquid stream is maintained so that the air is always saturated with liquid, the reading on the scale 22 will always be a. measure of the vapor pressure of the liquid, and it is only necessary in the refinery operation to control the product so that this reading is maintained at the predetermined value to give'the desired product of uniform quality. Also, knowing the value of Ha at which the apparatus is operated, the vapor pressure may be calculated at any time from Equation (9).

With regard to the liquid such as water which is introduced at l5, it is to be understood that this is done to automatically correct for the presence of any water in the liquid whose vapor pressure is being measured. .It is to be understood,

' mitting it to the apparatus.

It is to be understood that while we have herein described and illustrated one preferred method and apparatus for practicing our invention, that the inven'tion is not limited to the precise terms of such description but includes within its scope whatever departures fairly come within the spirit of the appended claims. It is to be further understood that the term orifice as used herein is to include all equivalent restricted passages such as short tubes, flow nozzles, Venturi throats and the like.

We claim as our invention: a

l. A method for the continuous determination of the vapor pressure of a liquid which comprises: passing a substantially constant stream of gas at a predetermined temperature through an orifice; measuring the pressure differential of the gas as it passes through the orifice; introducing a stream of said liquid into said stream of gas; bringing the gas and the vapor of said liquid to equilibrium at said predetermined temperature; separating the liquid from the'mixture of gas andvapor; passing the gas and vapor mixture through said orifice to measure the pressure differential thereof as an index of the change of density and volume; and determining the vapor pressure from a comparison of the initial and final pressure readings.

2. A method for determining vapor pressure which includes: passing a substantially constant ture of gas and vapor through said orifice; to-

measure the pressure differential thereof as an index of the change of density; and determining the vapor pressure from a comparison of the initial and final pressure readings.

3. A method for determining the vapor pressure of a liquid which contains a second liquid whose partial pressure is always equal to its vapor pressure, which comprises: passing a substantially constant stream of gas saturated with the vapor of. said second liquid at a predetermined temperature through an orifice; measuring the pressure differential of said gas and second liquid vapor across said orifice; saturating said gas and second vapor mixture with the vapor of the' first liquid;-passing the last mentioned gas and vapor mixture thi ,ugh said orifice to measure the pres-- sure differential thereof .as an index of the change of density and determining the vapor pressure from a comparison of the initial and final pressure readings; and calculating the vapor pressure of the first liquid from said pressure differential measurements.

4. An apparatus for obtaining vapor pressure measurements embodying: an equilibrium chamber; means for delivering a substantially constant stream of gas into said equilibrium chamber; means at the inlet of said equilibrium chamber for admitting a liquid whose vapor pressure is to be measured; a separating chamber at the outlet of said equilibrium chamber for separating the liquid from the gas and vapor mixture; a gase and vapor outlet from said separating chamber; an orifice in said gas and vapor outlet; and means for measuring a pressure dif-- chamber; means at the inlet of said equilibrium chamber for admitting a liquid whose vapor pressure is to be measured; a separating chamber at the outlet of said equilibrium chamber for separating the liquid from the gas and vapor rating chamber; an orifice in said gas and vapor; outlet; and means for measuring a pressure differential across said orifice.

6. An apparatus of the class described 'embodying: an elongated equilibrium tube; means including an orifice for delivering a substantially constant stream of gas into said tube;

means for delivering a liquid into the stream of a gas passing through said tube at one end thereof means comprising a separate chamber at the outlet endof said tube for separating the liquid from the gas and vapor mixture comingfrom said tube; a gas and vapor outlet connection to said separating means; a liquid outlet connection in the bottomv of said separating means; an orifice V 

